Composite structures, in particular, carbon fiber/resin materials, are rapidly increasing in use, and are of particular interest to the aerospace industry where there is a need for high strength-to-weight structures. A similar need exists in the navy and automobile industry where high-strength/light-weight bodies and other structural parts are being used for possible weight reduction for increased fuel efficiency. The technology involved in producing viable composite materials is quite complex with chemistry, physics and structural mechanics all making a contribution to the composites' properties. However, the overriding feature is the interaction between a carbon (graphite) fiber and a resin matrix at the nanophase level.
In composite laminates, the fiber and resin are essentially a physical blend of two basically dissimilar substances which, in an intimate mixture, result in the formation of a very unique load-carrying material. However, one of the major features in this laminate is the physical, or mechanical, bond that exists between these two dissimilar materials; and in order for the laminate to have any load-carrying capability, it is necessary for the resin to be in close proximity (usually mechanically locked) to the fiber. Thus, carbon/resin composite technology depends on the formation of a strong bond between a fiber substrate and a resin matrix; and the bond interaction parameters are analogous to those found in adhesive bonding processes.
Some of the parameters to be considered, then are: 1) adherend (fiber) surface, e.g., porosity, cleanliness and “wettability” (free-energy of the surface); 2) physical or chemical bonding involved in the adhesion to the fiber; 3) rheological properties of the matrix, e.g., viscosity; and 4) physical and mechanical properties of the substrate and the cured matrix, e.g., shear strength, compression strength, volume change during polymerization of the matrix and thermal coefficients of expansion, among others. Therefore, to optimize the fiber/resin interaction it is necessary to find the best condition for each of these parameters; and, of all of these, the degree to which physical or chemical interactions exist becomes the most critical to be studied. Carbon fibers, when received from the manufacturer, are normally coated with a sizing, e.g., polyvinylalcohol, which is there to keep the fibers from fraying or fuzzing prior to being impregnated with a resin for use in a composite. This sizing is not attached to the fiber, but exists as a sheath around the fiber. There is no chemical bond. Thus, when a resin is impregnated onto the fiber, the resin does not usually make any chemical bond to the fiber. This lack of a chemical bond is a weak link at the interface between the fiber and the matrix resin. This, in turn, affects the interphase between the fiber/resin interface and the bulk matrix.
A number of references exist that discuss the interactions between a fiber and the binding matrix; and it is claimed that the mechanical characteristics of a fiber/resin composite depend on the properties of the combined materials. Thus, of critical importance are the surface of the fiber, the nature of the fiber-resin bonding, and the mode of stress transfer at the interface. Factors that affect the fiber surface are the various pretreatments the fiber may be subjected to, such as nitric acid oxidation, and other oxidation and pyrolysis treatments. These, in effect, both increase surface area as well as create active sites for enhanced bonding between the fiber and the matrix. However, although various methods have been used to put functional groups on the fiber surface, these “active” sites are statistically sporadic (not completely uniform) on the entire surface. This, in effect, creates isolated sites of attachment and large amounts of resin attach in a discontinuous fashion. Thus, it was shown that by activating the surface of the fiber there was some control of the interface between the fiber and the resin; and, in measuring the failure modes it was found that two types of failure could occur, depending on the interfacial properties.
During the fabrication of a composite, it is essential to convert thousands of square inches of free fiber to a well-wetted, resin coated mixture. However, since the properties of the constituents, themselves, in the course of forming the composite, may be related to a variety of factors, such as, preferential surface adsorption, catalytic effects on the surface, chemical reactions between constituents and differential thermal effects, e.g., shrinkage or expansion, the interface is generally not examined in too great a detail, but, rather, more attention is paid to the interphase. This, not-withstanding, the general opinion is that a weak or strong bond at the interface governs the greater percentage of the properties of the composite.
As a matter of differentiation, therefore, the interface is usually one molecular layer thick, i.e., nanolayer, and the interphase is of macroscopic dimensions (as shown in FIG. 1); and it is the combination of the properties of the material in each phase that determines the behavior of a composite. Thus, it is the surface area and roughness of the reinforcement (fiber), the wetting properties of the matrix, and the differences in thermal and mechanical properties of the constituents that are strongly involved in determining the interaction, bonding and strength of a composite. For example, impregnating a fiber with a resin (either monomer, prepolymer or polymer), and subjecting the mixture to a curing reaction, i.e., polymerization and/or crosslinking, there is generally a shrinkage during the curing process due to a change in volume from a monomer (or prepolymer) to a high molecular weight, crosslinked polymer. And since the resin is not chemically bonded to the fiber, this shrinkage can cause the resin to either pull away from the fiber, either locally (in isolated sites) or totally, leaving a void between the matrix and the fiber; or it can compress onto the fiber and form a compression bond, called “frictional adhesion,” and this bond results in a bond strength of about 200 to 1000 lb./sq. in.
Thus, as has been indicated, on all other resin impregnation processes, even when the fiber surface has been activated to allow for some type of chemical bond, there is little or no complete chemical bond to the fiber, and there is no way to control the attachment such that only a nanolayer of resin is attached. It is a bulk, macroscopic process. With the electrodeposition, the process is controlled by time and voltage or amperage. Furthermore, the monomolecular layer of organic (or inorganic) compound (resin) may also function as a sizing that will protect the fiber from fraying or fuzzing. Thus, this process has a two-fold application. The present invention is a solution and a safe new material process application by modifying different resin compositions to create stronger covalent bonding on composite materials thereby creating stronger parts that represent a desired reduction of structural weight.